Diesel, among the various fuels derived from crude mineral oils, has some advantages such as good fuel efficiency, low cost and low carbon dioxide generation. On the other hand, there is a problem that the combustion of diesel produces a large quantity of air pollution. In order to solve these problems, various researches have been actively conducted on the bio-diesel as an alternative fuel of diesel. The bio-diesel has similar physical property to the traditional diesel oil, remarkably reduces air pollution, and is naturally recycling energy source. Generally, the bio-diesel is produced by transesterification reaction of vegetable oil such as rapeseed oil, soybean oil, sunflower oil, palm oil, etc, animal fats, waste-cooking oil, and so on with alcohol in the presence of acid catalyst or alkali catalyst. As the raw material of the source of bio-diesel, oil from the vegetables such rapeseed, soybean, sunflower, coconut palm etc. has been commonly used. Recently, the production of bio-diesel is rapidly and world-widely increasing so that the cost of the raw material for preparing bio-diesel also increases, which causes problems of food shortage and farmland demolition. Thus, there has been a demand on a method for preparing bio-diesel using a non-edible raw material.
On the other hand, crude oils or sludge oils which are produced during an oil-expelling step and include relatively large amount of fatty acid, acidulated soap stock (acid oils) which are produced as a by-product during an oil-refining step, greases which are discharged from households or restaurants etc. contain a large amount of impurities as well as a large amount of free fatty acids. Thus they are classified to representative non-edible oils. In detail, examples of the non-edible oils include i) sludge oils and waste oils separated from the waste water which is generated from the processes of cleaning, sterilization, crushing and so on, during the vegetable or animal oil-expelling process (oil extraction process), ii) crude oils including a large amount of fatty acid inherently, for example, including more than 3 weight % of fatty acid, such as microalgae oils or oils obtained from the rancid vegetables or animals, iii) acidulated soap stock (Soap stock, Acid oil) which is reduced to the fatty acid by acidification or soap-splitting of fatty acid salt (soap) generated at the neutralization process for removing fatty acids during an oil-refining step, iv) used cooking oil (yellow grease, fryer grease) including large amount of fatty acid, which is collected after high-temperature deep-frying with large amount of butter and oils at households or restaurants, v) trap grease (brown grease or FOG (Fats, Oils, Grease)) including large amount of fatty acid, which is separated from water at a grease trap which is installed in a drainpipe of a house or a restaurant, and so on. In case where the above-mentioned non-edible oils are used as the raw material for preparing the bio-diesel, it is difficult to prepare fuels satisfying the quality standard. Also, pre-treatment or refining reaction step is inevitable for removing fatty acid or for converting fatty acid to the bio-diesel. Thus, the reaction time and the reaction steps increase and the total manufacturing cost for preparing bio-diesel increases. Accordingly, it has been generally known that the fats containing large amount of fatty acids are not suitable as the raw materials for preparing bio-diesel.
As described above, the vegetable or animal fats and oils of triglyceride form are generally used as the raw materials for preparing the bio-diesel. However, when these raw materials include free fatty acids, the reaction efficiency is deteriorated. Thus, in order to increase the reaction efficiency and to improve the quality of the prepared fatty alkyl ester, several methods are developed. For example, in European patent publication No. 127104A, European patent publication No. 184740A and U.S. Pat. No. 4,164,506, and so on, a two-step method is disclosed in which the free fatty acids in oils and fats is first esterified and then the transesterification of oils and fats are carried out. In the methods, the esterification reaction is carried out by heating the mixture of fatty acid and fatty acid triglyceride with methanol at about 65° C. in the presence of sulfuric acid or sulfonic acid catalyst. European patent publication No. 708813A discloses a method for increasing the yield of fatty acid alkyl ester from oils and fats. In the method, the free fatty acid is separated from glycerin phase which is prepared by a transesterification reaction, and then the separated free fatty acid is esterified. In this method, the free fatty acid is obtained by the neutralization of glycerin phase, and the obtained free fatty acid is reacted for 2 hours at about 85° C. in the presence of strong sulfuric acid catalyst, which reduces the amount of fatty acid from 50% to 12%.
In U.S. patent publication No. 2011/0144375A, Bioresource Technology 102 (2011, 2380-2386) and so on, the yield of fatty acid alkyl ester is improved by reacting a mixture of free fatty acids, oils and fats with alcohol in the presence of a ceramic catalyst and by carrying out an esterification reaction and a transesterification reaction simultaneously. In this method, large amount of the catalyst is necessary and water produced at the esterification reaction and glycerin produced at the transesterification reaction cannot be effectively discharged to the outside of the reaction system. Thus, the conversion ratio of fatty acid into fatty acid alkyl ester is low and also complicate processes including neutralization, filtration, cleaning, distillation should be carried out for using the prepared fatty acid alkyl ester as the bio-diesel. In U.S. Pat. Nos. 6,855,838 and 7,795,460 and so on, it is disclosed a method of firstly converting the mixture of free fatty acid, oils and fats into fatty acids and then esterifying the converted fatty acids. In this method, the mixture of free fatty acid, oils and fats is converted into the fatty acids by hydrolysis in the presence of sulfuric acid or a solid acid catalyst and the converted fatty acids is esterified in the presence of sulfuric acid or on exchange resins.
In addition, methods for improving esterification reaction efficiency of fatty acid are disclosed, which utilize a mechanical apparatus or supersonic waves for causing dynamic turbulence in a reactor (Korean patent publication No. 2004-0101446, International Publication No. WO 2003/087278). In this method, the esterification is carried out by reacting the fatty acid and/or fatty acid contained in oils and fats with alcohol at a high pressure and a high temperature in the presence of sulfuric acid or ion exchange resin catalyst. Further, Korean patent publication No. 2004-87625 discloses a method for removing free fatty acid from waste cooking oil, using solid acid catalyst. The above mentioned methods commonly use an acid catalyst, such as sulfuric acid etc. If such an acid catalyst is not completely removed after the reaction, the quality of bio-diesel is deteriorated. Therefore, complicate processes for neutralizing, filtering, washing and cleaning the acid catalyst should be carried out. In addition, the reactor should have a corrosion resistance against the acid catalyst, which increases the cost of the production facilities. Also, the life cycle of the solid acid catalyst is generally short, and the cost for recycling the catalyst is high. Furthermore, in the above mentioned conventional methods, since the esterification of fatty acid is carried out at low temperature, water produced during the reaction is not efficiently removed to the outside of the reaction system. Thus, the conversion ratio of fatty acid into fatty acid alkyl ester is low, and the physical properties of the obtained fatty acid alkyl ester are not suitable for bio-diesel.